In a nonvolatile memory such as an NAND type EEPROM (Electrically Erasable Programmable Read-Only Memory), an inter-cell interference effect (proximity interference effect) is increased with the miniaturization of the memory. When an inter-cell interference effect is increased, the threshold voltage of a memory cell may be changed as a result of writing to an adjacent memory cell, thereby, causing the threshold voltage distribution of a memory cell to spread.
Meanwhile, a multi-valued memory cell such as an MLC (Multi Level Cell) has been developed. In the MLC, since the threshold voltage range corresponding to each stored data value is required to be relatively narrow, the threshold voltage distribution of a memory cell needs to be as narrow as possible. However, if the threshold voltage distribution of a memory cell is spread due to an inter-cell interference effect, it becomes difficult to achieve the threshold voltage ranges of the MLC. Therefore, it would be desirable to narrow the threshold voltage distribution of a memory cell while miniaturizing a device.